In variable displacement refrigerant compressors such as those of the variable angle wobble plate type used in motor vehicle air conditioning systems, it has been found desirable for extended compressor life expectancy to destroke to reduce the compressor displacement at high speed operation. Current practice with compressors of the above type is to control stroke with a suction pressure sensitive control valve which utilizes an evacuated bellows to regulate the pressure difference on the compressor pistons creating a force to move the wobble plate mechanism to the controlled stroke condition. The controlled stroke or displacement condition will under most operating conditions maintain the low side refrigerant pressure at a value that results in approximately maximum potential cooling performance with no risk of evaporator icing. A limitation of this concept is that this control criteria will, under high operating speed and air conditioning load conditions, result in a relatively high mechanism stroke condition that produces high mechanism inertial loads and lowers durability. The operating time at these high speed conditions is low and in turn the need for maximum air conditioning cooling is not priority over maintaining acceptable durability. In some cases in current practice the compressor is turned off via clutch disengagement at high speed conditions--a situation which protects the compressor but results in no cooling potential for the vehicle.
One proposed solution to the above problem is to add a solenoid valve that operates above a prescribed compressor speed to control a conventional stroke control valve so as to increase the crankcase pressure to discharge pressure to thereby effect minimum piston stroke and thereby minimum displacement to prolong compressor life. This proposal is disclosed in U.S Pat. No. 4,606,705 assigned to the assignee of the present invention.
A more desirable solution is a passive type destroker that would reduce the compressor displacement during certain high speed operation to improve the durability of the compressor mechanism while maintaining some compressor displacement and thereby air conditioning potential. Such a passive destroker is disclosed in U.S. Pat. application Ser. No. 204,338 filed June 9, 1988, and assigned to the assignee of the present invention. This device comprises a centrifugal destroke valve mechanism that is connected in parallel with a conventional stroke control valve arrangement and mechanically attached onto the compressor shaft so as to both slide and rotate in contact with an existing compressor part (i.e. the suction reed disk). The passive destroke mechanism includes a counterweighted valve member that rotates with the compressor shaft and at a predetermined trigger speed develops a centrifugal force that overrides a spring and friction force to slide the valve member from a closed to an open position. In the open position, a flow path is created between the discharge or high pressure side of the compressor and the crankcase to thereby allow a controlled discharge gas to bleed into the crankcase to destroke the compressor to a desired low displacement with the control effect accomplished by close control of the size of the delivery port. On the other hand, when the speed of the compressor is eventually reduced to the trigger speed, the centrifugal force is thereby reduced and overridden by the spring force so that the valve member then slides back towards its normal closed position wherein the compressor then operates as normal under the conventional stroke control valve arrangement. While this device has proven generally satisfactory, there remains a desire for a simpler and more durable passive destroker that can be incorporated in existing compressors with only minor modifications.